This invention relates to mechanical seals and in particular to heat exchangers for use in combination with such seals.
In general, it is well known to modify the temperature of certain components of a mechanical seal in order to provide a more efficient operation of the seal and/or longer seal life. Mechanical seals generate heat at the seal faces. It is also well known that the stability of the fluid film between the seal faces is critical for the satisfactory operation of the seal. The prevailing fluid film temperatures depend on the heat generated, the latter being a function of sealed pressure, shaft speed and coefficient of friction, and the heat dissipation properties of the seal including the heat transfer properties of the sealed fluid. Under certain conditions it may be necessary to introduce cooling to reduce the temperatures existing at the sealed faces. In other situations, as in the sealing of very heavy and viscous materials, it may be necessary to introduce a certain amount of heating thereby to achieve satisfactory conditions at the seal faces.
Many such thermosensitive fluids can have detrimental effects on other seal components, if temperatures are not closely controlled. Also, temperature limitations of secondary sealing components (eg. O-rings) can be exceeded resulting in premature failure.
Several traditional methods have been utilized in the past to modify the temperature of the seal chamber and/or mechanical seal components in an effort to ensure successful operation of the seal.
The first of these traditional methods involves the use of a jacket on the equipment's stuffing box to enable the circulation of a suitable heat exchange medium. These arrangements have a number of disadvantages. When considering the case of cooling, one must cool almost the whole casing, i.e. heat must be drawn away from the whole area. This requires a relatively large volume of coolant, and is consequently not very cost-effective or efficient.
Another method involves recirculation of the fluid being sealed through an external heat exchanger and back into the seal chamber. For example, fluid from the discharge side of a pump is used and this higher pressure fluid is injected into the seal chamber to prevent flashing at the seal faces. The combined effect of cooling the pumpage and increasing the sealed pressure often serves to prevent flashing in the sea interface. However, the increased pressure acts to increase the seal face closing force and this in turn increases the heat generated at the seal faces. Hence, the system has a built-in measure of inefficiency. Moreover, abrasive materials in the pumpage fluid may be forced into the sealing area. Also, an external heat exchanger is both expensive and relatively inefficient since it too often requires large volumes of heat exchange medium.
Another method involves the use of buffer fluid systems between double seals or quench and drain systems on the atmospheric side of a single seal. In the case of a buffer fluid, a heat exchange medium, separate from the fluid being pumped, is directed into the area between the two seals. The means for pumping or supplying this barrier fluid gives rise to additional expense. Even with double seal arrangements and a barrier fluid, the hot pumpage at the seal interface can give rise to flashing and dry seal faces. If the barrier fluid pressure is made sufficiently high as to avoid this problem, then relatively costly pumping equipment is needed. Another drawback is that one cannot readily detect inboard seal leakage with this system. Also, it too is relatively inefficient, requiring large volumes of heat exchange media either through the seal to the drain (in a "once-through" system) or through a coil in the convection tank (in a recirculating or "thermosiphon" closed-Loop system).
Another arrangement involves the use of a jacketed gland or a jacketed stationary seat designed to permit circulation of a heat transfer medium. Again, difficulties are encountered in that a relatively large gland plate must be heated or cooled before any cooling effect at the seal faces can be achieved. Furthermore, the design is such that there is a very lengthy heat transfer path from the jacketed gland to the seal interface. It is solely by means of conductive heat transfer through the stationary seal face and holder and/or gland plate that any cooling is effected. Since it has been shown that the greater percentage of heat dissipation (80 to 90 percent) naturally occurs through the rotating face, this points to another shortcoming of designs of this type. The result is that very large flow rates for the heat transfer media are required to effect any reasonable change in temperature at the seal faces. This technique also requires a fairly special design which cannot be applied easily to every situation.
Another variation involves simple recirculation of pumpage but without the use of an auxiliary heat exchanger. Thus, no cooling is effected; rather, at best, some percentage of the heat generated at the seal interface is dissipated. This technique again has most of the disadvantages associated with the second technique noted above.
It can be seen from the above that there is therefore a need for a very simple yet effective means for effecting heating or cooling at the seal faces and which means is applicable to a wide variety of seal designs.